Many nonwoven sheet materials, such as tissue webs, meltspun webs, hydroentangled webs, bonded carded webs, and the like are designed to include several important properties. For example, tissue products should have good bulk, a soft feel and should be highly absorbent. Tissue products should also have good strength even while wet and should resist tearing. Unfortunately, it is very difficult to produce a high strength tissue product that is also soft and highly absorbent. Usually, when steps are taken to increase one property of the product, other characteristics of the product are adversely affected. For instance, softness is typically increased by decreasing or reducing fiber bonding within the tissue product. Inhibiting or reducing fiber bonding, however, adversely affects the strength of the tissue web.
In the past, various methods have been used in order to increase the strength of nonwoven webs. For instance, many tissue webs and meltspun webs, such as meltblown webs and spunbond webs, undergo embossing or thermal bonding after being formed in order to increase the strength and integrity of the webs. In many applications, the web is thermally bonded according to a particular pattern. Nonwoven webs are also treated with latex materials to increase strength. The latex materials may be applied topically to the web in a pattern.
One particular process that has proved to be very successful in increasing the strength of tissue products, such as paper towels and wipers, without significantly adversely affecting other properties of the web is disclosed in U.S. Pat. No. 3,879,257 to Gentile, et al., which is incorporated herein by reference. In Gentile, et al., a process is disclosed in which a bonding material is applied in a fine, spaced apart pattern to one side of a fibrous web. The web is then adhered to a heated creping surface and creped from the surface. A bonding material is applied to the opposite side of the web and the web is similarly creped. The process disclosed in Gentile, et al. produces tissue products having exceptional bulk, outstanding softness and good absorbency. The surface regions of the web also provide excellent strength, abrasion resistance, and wipe-dry properties.
One problem that still persists, however, is the ability to produce a web that has properties in the cross-machine direction that are comparable to the properties of the web in the machine direction. For instance, many of the bonding patterns that are applied to webs typically greatly enhance the strength and stretch properties of the web in the machine direction without similarly increasing the same properties of the web in the cross-machine direction.
One particular problem encountered in the manufacture of nonwoven sheet materials is that most materials exhibit relatively high Poisson ratios in that when the material is pulled in the machine direction, the width of the material in the cross-machine direction significantly decreases. Since nonwoven materials are typically pulled in the machine direction during formation of the materials and during incorporation of the materials into a product, the above effect must be compensated for when processing the nonwoven materials. In some applications, the machine width is overdesigned or process conditions are compromised in order to control the width loss. This problem also extends into finishing operations causing issues of sheet control and overcompensating machine conditions or product dimensions.
As such, a need currently exists for a method of improving the overall properties of nonwoven sheet materials. In particular, a need currently exists for nonwoven sheet materials having a bonding pattern that reduces the Poisson ratio of the material such that the material has less tendency to shrink in the cross-machine direction when pulled in the machine direction.
A need also exists for an improved tissue product that possesses a negative Poisson ratio. When having a negative Poisson ratio, the web actually increases in width when the material is pulled in the lengthwise direction. It is believed that by creating a tissue product with a negative Poisson ratio, the product will also exhibit an increase in stretch in the cross-machine direction, with increased energy absorption in the cross direction.